The present invention is in the field of electronic image displays and more specifically relates to a method for fabricating high-contrast displays capable of reproducing digital radiography images.
In the last 20 years, tremendous improvements have been made in the image quality of cathode ray tubes and liquid crystal displays. Current displays are adequate for reproducing most color scenes, both as still images and video sequences (television). The spatial and temporal resolution of these electronic displays is commensurate with the resolution of the electronic capture devices (video and digital still cameras). The contrast ratio of these displays is also on par with the dynamic range of the electronic capture devices. Overall, these electronic displays compare favorably with non-electronic displays, such as photographic prints.
However, there are some applications for which electronic displays still cannot match the performance of traditional displays. One such application is radiography. Conventional radiography relies on silver halide films to capture, store and display X-ray images. A common use for radiography is medical diagnostic imaging. Chest X-rays (i.e. chest X-ray films), for instance, are used to capture and display 14″×17″ radiographic images with high resolution, high brightness and very high contrast. The resolution and brightness of the best and most expensive electronic displays is approaching the resolution and brightness of X-ray films viewed on a light box. Unfortunately, the contrast ratio of a CRT or LCD display is still very low compared to the contrast ratio produced by an X-ray film. An X-ray film is capable of reproducing contrast ratios up to 10,000:1 (optical density range up to 4) while a CRT or LCD display is only capable of reproducing contrast ratios up to 700:1. This is a very serious technical limitation, which is slowing the adoption of digital radiography among physicians.
Since CRTs and LCD panels cannot display the same number of gray scale levels as a conventional X-ray film, some of the diagnostic information in the image can be lost in the display process. To alleviate this problem, special software algorithms have been developed to compress the gray scale range of digital radiographs to fit the gray scale range of CRT and LCD displays. CRT and LCD displays are not capable of reproducing, in a linear fashion, all the gray scale information available in a digital radiograph, but they can reproduce enough grayscale levels to display a radiograph with acceptable image quality. A number of image processing techniques can be utilized to compress the dynamic range of digital radiographs. However, extreme precautions must be taken to avoid any undesirable artifacts caused by the gray scale compression techniques. Such artifacts (e.g. undershoot) could hide actual clinical information in a radiograph. The most popular image processing technique consists of performing hierarchically repeated unsharp masking steps. This technique is referred to as nonlinear multiscale processing or multiscale image contrast amplification.
Regardless of the actual technique used, the original gray scale information of the image is manipulated and the resulting displayed image looks different from a conventional X-ray film. Some experts argue that more pertinent information is visible on the displayed digital radiograph than on the X-ray film, since the gray scale range is reduced and therefore easier to be perceived by the human eye. Other experts argue that most radiologists have been trained in medical schools to review and interpret X-ray films, not digital radiographs, and should therefore be presented digital radiographs which look as close as possible to X-ray films. In this case, it would be desirable to have an electronic display capable of reproducing very high-contrast images similar to conventional X-ray images produced on film. Having such displays would alleviate the need for gray scale compression techniques and would allow radiologists to look at digital radiographs the same way they look at film radiographs.
Different methods have been suggested to increase the contrast ratio of LCD displays. Perry Penz describes one such method in U.S. Pat. No. 4,364,039 (“Penz”). Realizing that a single LCD panel has an inherently low contrast ratio, Penz suggests stacking multiple LCD panels on top of each other. The contrast ratio (defined as the maximum brightness divided by the minimum brightness) is indeed increased when two LCD panels are stacked against each other; the resulting contrast ratio is the contrast ratio of the first panel multiplied by the contrast ratio of the second panel.
FIG. 1 illustrates the theory described by Penz. Imaging device 100 includes rear LCD panel 105 and front LCD panel 110. Both panel 105 and panel 110 have contrast ratios of 100 to 1. Pixels 115 are represented by rectangles, with bright pixels 120 depicted as empty rectangles and dark pixels 125 as filled rectangles.
Light rays 130 and 135 are ideal, perfectly collimated rays from a source located to the right of rear LCD panel 105. Brightest pixel 140 of panel 105 and brightest pixel 145 of panel 110 transmit ray 130 at 100% of its original intensity. Darkest pixel 150 transmits ray 160 at 1% of the intensity of ray 135. Darkest pixel 155 of panel 110 transmits ray 165 at 1% of the intensity of ray 160. Accordingly, there is a theoretical contrast ratio of 10,000 to 1 between brightest pixel 145 and darkest pixel 155 of panel 110.
Even though this method does achieve the desired goal of increasing the overall contrast ratio, it introduces a number of undesirable artifacts caused by the parallax of light traversing both of the stacked LCD panels. In order to achieve an enhanced contrast ratio between neighboring pixels and remain properly “registered” with its corresponding pixel on the front LCD panel, all the light traversing a pixel on the back LCD panel would have to traverse the corresponding pixel, but not its neighbors. This parallax issue is more than a simple alignment issue between the two LCD panels; it is a fundamental problem caused by the fact that the two LCD panels cannot be infinitely close to each other and the backlight cannot be perfectly collimated.
This parallax problem is also illustrated in FIG. 1. Ray 170 traverses dark pixel 175 of panel 105 and corresponding dark pixel 180 of panel 110. Therefore, pixels 175 and 180 are properly registered, as in the ideal case described above. However, ray 185 is not perfectly collimated and therefore does not traverse both bright pixel 190 and corresponding bright pixel 182 before reaching observer 195. Instead, ray 185 traverses bright pixel 190 and a portion of dark pixel 180. Similarly, ray 192 does not traverse bright pixel 194 and corresponding bright pixel 184. Instead, ray 192 traverses bright pixel 194 and a portion of dark pixel 180. Therefore, pixel 190 is not properly registered with pixel 192 and pixel 194 is not properly registered with pixel 184.
Compounding this problem is the fact that in the real world, observer 195 is at a finite distance from the stacked display assembly and therefore sees different parts of the display from different angles, as shown in FIG. 2. Ray 200 passes through pixel 201 and corresponding pixel 202 before reaching observer 195. Accordingly, pixels 201 and 202 are properly registered. Pixel 206 corresponds with pixel 207, but ray 205 traverses pixels 206 and 208. Pixel 211 corresponds with pixel 212, but ray 210 traverses pixels 211 and 213.
Even if the parallax problem could be solved, the alignment problem would remain critical as far as creating a perfect registration between the pixels from the two LCD panels. It is clear that it would be desirable to have an electronic display capable of reproducing a very high contrast ratio but without the parallax and alignment problems mentioned above.